THE PROCESS OF CLEAVAGE. 51 



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der Richtungskorper. Zool. Jahrbucher. Bd. III. Abth. f. Morph. 



1889, p. 515. 

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sowie iiber Reifung' u. Befruchtung desselben. Zool. Jahrbucher. Bd. IV. 



Abth. f; Morph. 1889. 

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Fecundation. Jour. Morphol. Vol. I. 1887. 

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Eies. Biol. Centralblatt. Bd, VII. 1888, p. 659. 



CHAPTER III. 

 THE PROCESS OF CLEAVAGE. 



FERTILISATION is in most instances immediately followed by further 

 development, which begins with the division of the egg-cell the 

 simple elementary organism into an ever-increasing number of 

 small cells the process of cleavage. We shall begin the study of 

 cleavage with a very simple case, and here also choose as a foundation 

 for the presentation of the subject the egg of an Echinoderm and 

 the egg of the common Ascaris of the Horse. 



In the living egg of the Echinoderm the cleavage-nucleus (fig. 26 

 //t), which arose from the fusion of egg-nucleus and spermatic 

 nucleus, is at first spheroidal, and lies exactly in the middle of the 

 egg, where it forms the centre of a radiation which affects the 

 whole yolk-mass ; but it soon begins to be slightly elongated, and 

 at the same time to become less and less distinct, so that with the 

 living object one might be misled into assuming that it had been 

 completely dissolved. Before this, veiy regular changes in the dis- 

 tribution and arrangement of the protoplasm around the nucleus 

 have taken place. The monocentric radiation resulting from fer- 

 tilisation is divided. The two r.ewly formed radiations thereupon 

 move to the poles of the elongated nucleus. At first small and in- 

 significant, they rapidly extend, and finally each occupies a half of 

 the egg (fig. 27), and the rays of the two systems meet at a sharp 

 angle in the median plane of the egg. 



Just in proportion as the two radiations become more distinct> 

 there arises, within the granular yolk, as the starting-point and/ 



